Re: [PATCH v2] repack: Add option to preserve and prune old pack files

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 12, 2017, 18:03 UTC
Message-ID: <xmqqbmt6v0cf.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20170312122418.e544nrmqniycqouc@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

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> I can think of one downside of a time-based solution, though: if you run
> multiple gc's during the time period, you may end up using a lot of disk
> space (one repo's worth per gc). But that's a fundamental tension in the
> problem space; the whole point is to waste disk to keep helping old
> processes.

Yes. If you want to help a process that mmap's a packfile and wants to keep using it for N seconds, no matter how many times somebody else ran "git repack" while you are doing your work within that timeframe, you somehow need to make sure the NFS server knows the file is still in use for that N seconds.

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> But you may want a knob that lets you slide between those two
> things. For instance, if you kept a sliding window of N sets of
> preserved packs, and ejected from one end of the window (regardless of
> time), while inserting into the other end. James' existing patch is that
> same strategy with a hardcoded window of "1".

Again, yes. But then the user does not get any guarantee of how long-living a process the user can have without getting broken by the NFS server losing track of a packfile that is still in use. My suggestion for the "expiry" based approach is essentially that I do not see a useful tradeoff afforded by having such a knob.

> The other variable you can manipulate is whether to gc in the first
> place. E.g., don't gc if there are N preserved sets (or sets consuming
> more than N bytes, or whatever). You could do that check outside of git
> entirely (or in an auto-gc hook, if you're using it).

Yes, "don't gc/repack more than once within N seconds" may also be an alternative and may generally be more useful by covering general source of wastage coming from doing gc too frequently, not necessarily limited to preserved pack accumulation.